To investigate the pathogenesis of dystrophic cardiac calcification in mice, we studied myocardial and skeletal muscle (diaphragm) necrosis induced by freeze-thaw injury through the abdominal portion of the diaphragm in DBA/2, C3H/He, and C57BL/6 (control) mice. Two mice from each mouse strain were euthanized 6, 12, 24, and 36 h after the initial freeze-thaw injury; 6 mice from each strain were euthanized 2, 4, 7, 14, and 28 days after injury. The hearts and diaphragms were studied by light and electron microscopic techniques. Myocardial and diaphragmatic mineralization in response to injury occurred only in DBA/2 and C3H/He mice and was present as early as 2 days after initial myocyte injury. Ultrastructurally the mineralized deposits first accumulated in mitochondria as early as 24 h after injury, with subsequent complete mineralization of the mitochondria and surrounding sarcoplasm by 48 h. These results suggest that the pathogenesis of dystrophic cardiac calcification in DBA/2 and C3H/He mice may be related to disturbed myocyte calcium metabolism, leading to mitochondrial calcium overload and myocardial calcification.